This invention relates to an imaging apparatus for representing an image of concentration ratios between hemoglobin and oxyhemoglobin in blood, with different measuring values being represented with different colors and/or gray shades, comprising a light source capable of irradiating the object with light, which light comprises at least three wavelengths xcex1, xcex2 and xcex3, wherein xcex1 is in the wavelength range of 600 to 700 nm, with a preference for 660 nm, xcex2 is in the wavelength range of 900 to 1000 nm, with a preference for 940 nm, and xcex3 is in the wavelength range of 790 to 830 nm, with a preference for 810 nm; detection means for at least detecting the intensity of light emitted by the object at the respective wavelengths xcex1, xcex2 and xcex3, resulting in detection signals S1, S2 and S3 ; a processing unit for calculating an optical image of the pattern of concentration ratios, from the respective signals S1, S2 and S3; display means for displaying the calculated optical image
Such an apparatus as described in the preamble is known from U.S. Pat. No. 5,318,022. In this patent specification, it is described how with a narrow light beam consisting of three wavelengths a human eye is scanned, malting use of the different absorption behavior of blood at different wavelengths. This is caused by the different absorption characteristics of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb). The wavelengths are selected such that light of a first wavelength xcex1 sustains a relatively low absorption in oxygen-rich blood, while light of a second wavelength xcex2 is absorbed relatively strongly in oxygen-rich blood, both in relation to an absorption in oxygen-poor blood. The so-called isobestic wavelength xcex3 is the wavelength at which no difference in absorption occurs with respect to oxygen-poor blood. This wavelength xcex3 lies between the first and second wavelengths xcex1 and xcex2 and serves as reference.
Of interest, in practice, is the concentration ratio of those two substances, represented formulaically by             HbO      2              Hb      +              HbO        2              xc3x97  100  ⁢      %    .  
It is clear that the apparatus according to the known technique is not suitable for making optical images with a high resolution, both in the time domain and in the place domain. An intrinsic limit on this resolution is imposed in that the images are built up by sequentially scanning a grid-shaped pattern with a narrow beam, from which an image is derived through synchronization. Clearly, such an apparatus can only yield images of limited grade. In particular, the apparatus is not suitable for producing a virtually instantaneous image of a region of interest of an object, which can be analyzed with sufficient resolution in time.
The object of the invention is to enable a reliable detection of the oxygen content in blood, which can be performed virtually continuously in time. To that end, the invention provides an imaging apparatus as described in the preamble, wherein the apparatus is further characterized according to characterizing part of claim 1. This renders sequential scanning of a grid pattern superfluous, so that the feasible image frequency increases by a factor equal to the square of the desired number of pixels/inch. Thus, virtually instantaneous measuring results over a whole region of interest can be obtained. The combination of an image of visible light with the image of the oxygen concentration levels and/or the vascular system (called SpO2 image for short) provides advantages in the application of the invention in probe examination, the probe then being provided with the apparatus according to the invention. By reading out the images of visible light, steering can be roughly controlled, while through the measuring signals in situ au accurate picture of the vascular system and/or the oxygen concentrations therein can be obtained. The image of visible light and the pattern of concentrations or concentration changes can be projected in a single overlapping image, which improves orientation in the body in manipulating the probe.
In a preferred embodiment, the light signals S1, S2 and S3 have a characteristic modulation. What is thus achieved is that the apparatus can be made insensitive to non-modulated signals, such as ambient light. Also, if the sensors in the apparatus have a sensitivity to different wavelength bands, the contributions of the different wavelengths to the output signal of the apparatus can be determined by demodulation of the signal. Modulating the light signal, finally, affords the possibility of raising the light intensity to a maximum without this leading to distortion. The applications hereof will be further elucidated in the description of the drawings.
In a further preferred embodiment, the imaging apparatus is suitable for determining measurements consecutive in time. The apparatus may further comprise means for analyzing the measuring values. Relevant parameters can be determined and analyzed, such as the time-average value and deviation, minimum and maximum, as well as, given an assumed cyclic change of the concentration ratio, the spectral features of the waveform.
In an application of the invention as a detector for determining the ratio of hemoglobin and oxyhemoglobin in blood, with the first and second substance being hemoglobin and oxyhemoglobin, respectively, the signal sources are so arranged that the first wavelength xcex1 is in the wavelength range of 600 to 700 nm, with a preference for 660 nm, the second wavelength xcex2 is in the wavelength range of 900 to 1000 nm, with a preference for 940 nm, and the third wavelength xcex3 is in the wavelength range of 790 to 830 nm, with a preference for 810 nm.
Preferably, the apparatus further comprises means for analyzing the measured values numerically. Such means can be image analysis means. They may also be means that represent the value of the reference signal S3, thus yielding an image of the light absorption that is independent of the oxygen level. This reflects the actual vascular volume, which enables plethysmographic measurements to be performed. The means can comprise the analysis means mentioned earlier, as well as the possibility of determining region averages and the time development of such parameters. The means may further comprise indicating means for defining such a region of interest, such as a light pen, tracker ball or mouse indication. Also, the analysis means may comprise the possibility of comparing the measuring values with reference values, or with the values measured in corresponding parts in the left-side or right-side parts of the body.
The measuring signals S1, S2 and S3 can be distinguished from the visible light by modulation technique.
The invention further relates to an image observing apparatus, such as a camera or endoscope with an imaging apparatus as described above.